Triumph BSA 500 650 Pre-Unit and Unit Main Bearing Hepolite MRJ 1-1/8inch C3 PN# 70-1591C3 H

The Hepolite line of reproduction parts for vintage Triumph and BSA motorcycles now includes the ubiquitous MRJ 1-1/8 open Imperial deep groove ball bearing in C3 clearance. This crankshaft main bearing was used in both drive-side and timing-side applications in many Triumph and BSA 500 and 650 engines through 1971. Although the 70-1591 part number is listed for fitment in the Triumph T150/T160 models, the C3 clearance is not recommended for use as the drive-side crankshaft bearing. Not compatible with Triumph 750 twins.

This UK-made MRJ 1-1/8 C3 ball bearing is most commonly associated with pre-unit Triumph 500 and 650 models in both drive-side and timing-side crankshaft applications, as well as unit-construction Triumph 650 engines through 1965. From 1966 through mid-1971 (to engine number GE27029), this bearing was used on the Triumph 650 timing-side crankshaft. BSA specified the MRJ 1-1/8 bearing for the 1962-1965 twin-cylinder models on the drive-side of the crankshaft and listed it as 67-1240. The ID is 1.125″, with a 2.8125″ OD, and a width of .8125″.

The C3 clearance refers to the internal diametral clearance of the bearing and is important to consider when ordering bearings for an engine overhaul. CN clearance is normal clearance, C2 is less clearance than CN, and C3 clearance has more clearance than CN. The MRJ 1-18 main bearing supports the crankshaft on one or both ends, depending on the application. While a very robust bearing that offers a long service life, they are typically replaced during an engine overhaul due to the damage inflicted to the bearing during removal.

The bearing is typically frozen for several hours before being dropped into the heated engine case, minimizing the need for drifting it into position in the bore. Replacing crankshaft main bearings requires patience and skill and may be best left in the hands of a professional mechanic for best results.